What matters most
Mono County requires an OWTS permit before septic-system construction. Its current process calls for an application, fee, to-scale plot plan, simple site-location map, site evaluation, and the test trenches or percolation testing required for the parcel.
Key takeaways
- County guidance describes typical conventional-site criteria, but they are screening conditions—not automatic approval.
- The construction guide uses two test trenches and two percolation tests as a general starting point while stating that requirements vary by area.
- The reserve area must equal 100% of the initial leach-field area.
- Mono County states that seepage pits and cesspools are prohibited.
What to include with the application
Mono County guidance calls for a plot plan showing applicable water sources, streams, ponds, lakes, canals, drainage, buildings, drives, parking, septic tank and leach field, elevations or grade, nearby wells and water, access, property identification, and a location map.
A simple sketch may be acceptable when it is legible, to scale, and complete. Sloping, constrained, or complex sites may need more detailed mapping so the testing and system can be coordinated with the development.
Test trenches and percolation testing
The County construction guide uses two ten-foot test trenches and two percolation tests in the proposed disposal area as a general starting point. It also states that requirements vary: testing may be waived in some areas or expanded where conditions warrant.
Where percolation testing is required, the guide identifies a registered engineer, registered geologist, or registered environmental health specialist as acceptable. Confirm the site-specific scope with Environmental Health before excavation.
Conventional-system site considerations
Mono County’s current OWTS page lists typical acceptable percolation of 5 to 60 minutes per inch, at least five feet of separation below the trench to groundwater or bedrock, a reserve area equal to 100% of the initial leach field, slope below 30%, and required separations from wells, structures, water, and other features.
A test rate inside the typical range does not override a limiting soil profile, coarse rock, groundwater, bedrock, slope, setbacks, or inadequate replacement area.
Seepage pits, commercial systems, and alternatives
Mono County states that seepage pits and cesspools are prohibited, so a deep pit should not be assumed to solve a constrained leach-field layout. Alternative systems require additional site-specific review and professional design.
The County requires commercial systems to be designed by an appropriately licensed professional and requires Environmental Health verification of soil testing.
Coordinate the required inspections
County materials identify a site and soil-profile review before permit issuance, an open-trench inspection before rock and pipe placement, and a final inspection of the tank, leach lines, and distribution components.
Schedule inspections before covering work. Contractor construction sequencing should follow the approved plan and current permit conditions.
What to send Calicore for a proposal
Provide the address or APN, proposed structure and bedroom count, water source, available survey or site sketch, slope and access information, prior septic records, and County direction. Photographs help identify equipment and seasonal access needs.
Calicore can define a scope for septic soil testing, OWTS support, design-level topographic mapping, grading and drainage, and permit coordination.
Mono County mountain and high-desert sites can combine snow, slope, rock, groundwater, short construction seasons, and access constraints. Plan the field work around the complete site—not just a test date.
This page provides general educational information based on agency materials available when reviewed. Septic, grading, building, and wastewater requirements can change and may vary by parcel, jurisdiction, proposed use, site conditions, and agency interpretation. The permitting agency determines the required scope and whether a project is approved. Confirm current requirements before testing, design, excavation, or construction.
Frequently asked
Questions about this topic
Does Mono County always require two perc tests?+
The County guide uses two as a general starting point but states that testing may be waived or increased depending on area and site conditions.
How deep are Mono County soil-profile trenches?+
The construction guide generally describes two ten-foot test trenches. Environmental Health determines the actual site-specific requirement.
Can I use a seepage pit if the parcel is too small for leach lines?+
Mono County states that seepage pits and cesspools are prohibited. Another approved alternative would require County review and professional design.